This invention relates to the art of wireless communication; and more particularly, it relates to antenna feed structures which transmit and receive electromagnetic waves in the C, X, and Ku frequency bands from a single I/O port.
By the C-band is herein meant the set of frequencies which range from 3.625 GHz to 6.425 GHz. Likewise, by the X-band is herein meant the set of frequencies which range from 7.250 GHz to 8.40 GHz; and by the Ku-band is herein meant the set of frequencies which range from 10.950 GHz to 14.500 GHz.
In the prior art, the C, X and Ku frequency bands have been used to communicate from a ground station on Earth to a geosynchronous satellite and back to another ground station on Earth. Each of the bands C, X, Ku are normally subdivided into many sub-bands of about 36 MHz to 210 MHz; and in each such sub-band, one or more data streams are transmitted and/or received.
To transmit one data stream, a data signal which may be digital or analog, is first sent to a modulator circuit in a ground station. There, the data signal modulates a carrier signal whose frequency lies within a certain sub-band. Then the modulated carrier signal is sent to an input port on a waveguide assembly, which is commonly called an antenna feed. This antenna feed acts as a transducer that converts the modulated carrier to radiated electromagnetic waves at an input/output port (I/O port). From the I/O port, the waves are directed by one or more reflectors at the ground station to the geosynchronous satellite.
To receive one data stream, the above process occurs in reverse. That is, radiated electromagnetic waves from the satellite are first directed by the reflectors at the ground station into the I/O port of the antenna feed. Then, the antenna feed acts as a transducer to route the received waves to appropriate receive ports. From there, the modulated carrier is then demodulated to recover the data stream.
However, a major limitation with the prior art is that in order to transmit/receive in all three of the frequency bands C, X and Ku, three physically separate antenna feed structures are needed. That is, a C-band antenna feed with its own I/O port is needed for transmitting/receiving in the C-band; an X-band antenna feed with its own I/O port is needed for transmitting/receiving in the X-band; and a Ku-band antenna feed with its own I/O port is needed for transmitting/receiving in the Ku-band.
Since three separate antenna feed structures are needed, it follows that the data transmission/reception from one parabolic reflector can occur only in one frequency band at a time. For example, before data transmission/reception can occur in the C-band, the C-band antenna feed must be physically moved such that its I/O port is located at the focal point of the parabolic reflector. Then, to switch data transmission/reception to the X-band, the C-band antenna feed must be physically moved out of the focal point of the reflector and X-band antenna feed must be physically moved to the focal point of the reflector. Consequently, the number of data streams which can be transmitted/received simultaneously is limited to the number of data streams which fit into one frequency band.
Also, having to physically move the C-band, X-band and Ku-band antenna feed structures to and from the focal point of the reflector is a time-consuming and tedious operation. However, if the movement is not done accurately, misalignment problems between the reflector and the I/O port of the antenna feed structure will occur.
Specifically, when the I/O port of an antenna feed is misaligned with its reflector, the radiation pattern of the transmitted electromagnetic waves will be distorted. In turn, this distortion can interfere with transmissions from any other independent sources. Thus, after a switch is made from one antenna feed to another, tests must be rerun to obtain actual radiation patterns, and those radiation patterns must be recertified by some organization such as the FCC, INTELSAT, EUTELSAT, AND PANAMSAT. This process takes days to complete. Consequently, practically all ground stations limit their transmissions/receptions to just one of the three bands C, X, and Ku.
Accordingly, a primary object of the present invention is to provide a novel structure for a multi-band antenna feed by which all of the above drawbacks are overcome.